In the past year our principle ongoing project to determine the function of tubulin modification by reversible enzymatic addition of C-terminal tyrosine has been temporarilly suspended. In continuing last year's investigation of a new brain protein that aggregates tubulin, the observation that precipitation was accompanied by a stoichiometric burst of GTP hydrolysis (such as accompanies microtubule polymerization) supported the possibility that this protein might have a specific function, such as blocking indiscriminate assembly. This has now become less likely from the observations that a) the basic fraction from brain cytosol contains more than 1 tubulin aggregating protein and b) polylysine produces most of the same effects. To begin a study of the effects of protein phosphorylation on the functions of MAP-2 (the principle microtubule-associated protein of brain) we purified a non-specific brain protein phosphatase. With the latter + cAMP-dependent protein kinase we have prepared MAP-2 containing from 2 to 18 moles of phosphate per mole protein.